EPIGENETIC MODIFICATIONS TO CYTOSINE AND ALZHEIMER’S DISEASE: A QUANTITATIVE ANALYSIS OF POST-MORTEM TISSUE

• Main Author: Ellison, Elizabeth M.
• Format: Others
• Published: UKnowledge 2017
• Subjects: cytosine; 5-hydroxymethylcytosine; 5-methylcytosine; Alzheimer's disease; epigenetic modifications; quantification; Analytical Chemistry
• Online Access: http://uknowledge.uky.edu/chemistry_etds/86; http://uknowledge.uky.edu/cgi/viewcontent.cgi?article=1092&context=chemistry_etds

Alzheimer’s disease (AD) is the most common form of dementia and the sixth leading cause of death in the United States, with no therapeutic option to slow or halt disease progression. Development of two characteristic pathologic lesions, amyloid beta plaques and neurofibrillary tangles, in the brain are associated with synaptic dysfunction and neuron loss leading to memory impairment and cognitive decline. Although mutations in genes involved in amyloid beta processing are linked to increased plaque formation in the inherited familial form of AD, the more common idiopathic form, termed sporadic AD, develops in the absence of gene mutations. In contrast, alterations in gene expression and transcription occur in plaque and tangle susceptible brain regions of sporadic AD subjects, even in the earliest stages of development of pathologic burden, and may give insight into the pathogenesis of AD. Epigenetic modifications to cytosine are known to alter transcriptional states and gene expression in embryonic development as well as in cancer studies. With the discovery of enzymatically oxidized derivatives of 5-methylcytosine (5-mC), the most common epigenetic cytosine modification, a probable demethylation pathway has been suggested to alter transcriptional states of DNA. The most abundant 5-mC derivative, 5-hydroxymethylcytosine (5-hmC), while expressed at low concentrations throughout the body, is expressed at high concentrations in brain cells. To determine the role cytosine modifications play in AD, this study was directed at the quantification of epigenetic modifications to cytosine in several stages of AD progression using global, genome-wide, and gene-specific studies. To determine global levels of each cytosine derivative in brain regions relevant to AD progression, a gas chromatography/mass spectrometry quantitative analysis was utilized to analyze cytosine, 5-mC, and 5-hmC in tissue specimens from multiple brain regions of AD subjects, including early and late stages of AD progression. To determine the genome-wide impact of 5-hmC on biologically relevant pathways in AD, a single-base resolution sequencing analysis was used to map hydroxymethylation throughout the hippocampus of late stage AD subjects. Finally, to determine gene-specific levels of cytosine, 5-mC, and 5-hmC, a quantitative polymerase chain reaction (qPCR) protocol was paired with specific restriction enzyme digestion to analyze target sequences within exons of genes related to sporadic AD. Results from these studies show epigenetic modifications to cytosine are altered on the global, genome-wide, and gene-specific levels in AD subjects compared to normal aging, particularly in early stages of AD progression, suggesting alterations to the epigenetic landscape may play a role in the dysregulation of transcription and the pathogenesis of AD.